Properties and dynamics of the σ(M′-Re) π* excited state of photoreactive dinuclear LnM′-Re(CO) 3 (α-diimine) (LnM′ = Ph3Sn, (CO)5Mn, (CO)5Re; α-diimine = bpy′, iPr-PyCa, iPr-DAB) complexes studied by time-resolved emission and absorption spectroscopies

Article abstract of DOI:10.1016/s0020-1693(96)05203-6

The photophysics and photochemistry of the metal-metal bonded complexes L_nM′Re(CO)₃(α-diimine) (L_nM′ = Ph₃Sn, (CO)₅Re, (CO)₅Mn; α-diimine = bpy′, iPr-PyCa, iPr-DAB) have been studied. According to the time-resolved emission (80 K) and absorption (room temperature) spectra, the lowest excited state has a ³σ (M′-Re) π* character. It is a bound state, which can only be populated by surface crossing from optically excited MLCT states. Homolysis of the metal-metal bond from the σπ* state is promoted by nucleophilic and chlorinated solvents. Exceptional in this respect is the complex Ph₃SnRe(CO)₃(bpy′), which is nearly photostable in non-chlorinated solvents. The lifetime of the ³σπ* state decreases in the order α-diimine = bpy′ > iPr-PyCa > iPr-DAB > pAn-DAB. This trend is mainly determined by the energy gap law. The L_nM′ dependence is more complicated because of an additional deactivating effect of an excited state distortion which depends on L_nM′. At 80 K, the lifetime is determined by the weak coupling to the ground state; at room temperature by dissociation of M′-Re (with the exception of Sn-Re).